Structural isomers are a core concept in IB Chemistry Topic 10 (Organic Chemistry). They explain why molecules with the same molecular formula can behave differently, look different, and react in different ways. Understanding structural isomerism helps you identify organic compounds, predict reactions, and decode exam questions involving molecular structures.
What Are Structural Isomers?
Structural isomers are compounds that share the same molecular formula but differ in the arrangement of their atoms.
This means:
- They contain the same number of each type of atom.
- They are connected differently.
- They have different physical and chemical properties.
Because structure determines function in organic chemistry, even small changes in connectivity can produce major differences.
Example:
C₄H₁₀ can be:
- Butane
- Methylpropane
Same formula, different structure → different properties.
Why Structural Isomers Exist
Carbon can form:
- Long chains
- Branched chains
- Ring structures
- Multiple bond arrangements
Its bonding versatility allows many different ways to arrange atoms while keeping the formula constant. As the number of carbons increases, the number of possible isomers grows exponentially.
Types of Structural Isomerism
IB Chemistry divides structural isomers into three main categories:
- Chain isomers
- Position isomers
- Functional group isomers
Each type reflects a different kind of rearrangement.
1. Chain Isomers
Chain isomers differ in the arrangement of the carbon skeleton.
The same atoms can form:
- Straight chains
- Branched chains
Example: C₅H₁₂
- Pentane
- Methylbutane
- Dimethylpropane
Effects of branching:
- Lower boiling points (weaker London forces)
- More compact structure
- Slight differences in shape and reactivity
Chain isomerism appears in alkanes, alkenes, and some alcohols.
2. Position Isomers
Position isomers have the same carbon skeleton and the same functional group, but the functional group is located in a different position.
Example: C₃H₇Cl
- 1-chloropropane
- 2-chloropropane
Another example: C₄H₉OH
- 1-butanol
- 2-butanol
Even though the functional group is the same, moving it along the chain changes:
- Polarity
- Boiling point
- Reactivity in mechanisms
Position isomerism commonly appears in:
- Alcohols
- Halogenoalkanes
- Alkenes
- Ketones
3. Functional Group Isomers
Functional group isomers have the same molecular formula but completely different functional groups.
This type produces the largest differences in properties.
Examples:
C₂H₆O
- Ethanol (alcohol)
- Dimethyl ether (ether)
C₃H₆O
- Propanal (aldehyde)
- Propanone (ketone)
Functional group isomers behave very differently:
- Different boiling points
- Different IR/NMR spectra
- Different chemical reactions
This type of isomerism is highly tested in IB exams.
Why Structural Isomers Have Different Properties
Structural changes affect:
- Intermolecular forces
- Shape and compactness
- Polarity
- Boiling and melting points
- Reactivity
- Solubility
For example:
- Branched alkanes have lower boiling points than straight-chain alkanes.
- Alcohols have higher boiling points than ethers because they form hydrogen bonds.
- Aldehydes oxidize easily; ketones do not.
These differences make structural isomers chemically distinct compounds.
Structural Isomers vs Stereoisomers
IB students often confuse these.
Structural isomers:
- Different connectivity
- Completely different structures
Stereoisomers:
- Same connectivity
- Different spatial arrangement
(e.g., cis–trans or optical isomers)
Structural isomers must be identified before considering stereochemistry.
How to Identify Structural Isomers in IB Exams
- Count carbons and hydrogens to ensure they match the formula.
- Check the carbon skeleton for different branching.
- Move the functional group and see if it creates a new molecule.
- Consider alternate functional groups if the formula allows functional group isomers.
- Draw systematically to avoid duplicates.
Being organized prevents missing possible isomers.
Common IB Misunderstandings
“Isomers must look completely different.”
Not always. Some differ only by the position of one substituent.
“Moving a group by one carbon doesn’t create a new isomer.”
It does—if the connectivity changes.
“Functional group isomers behave similarly.”
They often behave very differently because functional groups control reactivity.
FAQs
How do I know when two structures are different isomers?
If their connectivity differs—even slightly—they are structural isomers.
Can structural isomers have the same boiling point?
Rarely. Differences in structure usually change intermolecular forces.
Do all organic molecules have isomers?
Small molecules may not, but most with four or more carbons do.
Conclusion
Structural isomers share the same molecular formula but differ in how their atoms are connected. The three main types—chain, position, and functional group isomers—explain why organic compounds with identical formulas can have drastically different physical and chemical properties. Mastering structural isomerism is essential for predicting behavior and solving organic chemistry problems in the IB program.
